Piezoelectric actuator

ABSTRACT

The invention relates to a piezoelectric actuator ( 1 ) comprising a stack of a plurality of individual piezoelectric actuator elements ( 2, 2′, 2 ″) which are arranged between inner electrodes ( 3, 3′, 3 ″). Said piezoelectric actuator comprises a first metallisation strip ( 4 ) and a second metallisation strip ( 5 ), the inner electrodes ( 3, 3′, 3 ″) being respectively connected to the first or second metallisation strips ( 4, 5 ) in an alternating manner. A first outer electrode ( 6 ) and a second outer electrode ( 7 ) are fixed to the first or the second metallisation strips ( 4, 5 ) in order to electrically contact the piezoelectric actuator ( 1 ). Said outer electrodes ( 6, 7 ) respectively comprise a connection element ( 8, 9 ) for externally contacting the piezoelectric actuator ( 1 ), and at least one region which is embodied in such a way that it compensates length variations of the piezoelectric actuator ( 1 ) in the main oscillation direction ( 10 ) as a result of its design and arrangement. The elastic deformation exclusively extends inside a plane which is parallel to the main oscillation direction ( 10 ).

The invention relates to a piezoelectric actuator according to thepreamble of claim 1.

Generic piezoelectric actuators are often used to control injectionvalves in motor vehicles.

Piezoceramics have the property of expanding or contracting when anelectric voltage is applied, depending on the polarity of the voltage.This effect is utilized in piezoelectric actuators. However, theeffective linear expansion is less than 0.2 percent of the layerthickness of the piezoceramic along the principal axis. At the sametime, the electric voltage necessary to operate the piezoelement risesas the layer thickness of the piezoelement increases. In order to keepthe voltage within bounds and at the same time to achieve technicallymeaningful linear expansion, a plurality of piezoceramic layers areusually arranged on top of each other. The individual piezoceramiclayers, which hereinafter are referred to as piezoelectric actuatorelements, are provided with metal inner electrodes on both sides forpolarization.

A piezoelectric actuator which has two metallization strips toelectrically contact the piezoelectric actuator elements on its outerside is already known from DE 196 48 545 A1. The inner electrodes arerespectively connected in an alternating manner to one of the twometallization strips and electrically switched in parallel via outerelectrodes. To this end, the outer electrodes have connection elementsfor externally contacting the piezoelectric actuator.

On account of the high dynamic load of the piezoelectric actuator,cracks may occur in the ceramic. The metallization strip may be split asa result. On the cracked edge this leads to arcing which leads todestruction of the piezoelectric actuator.

To avoid failures of the piezoelectric actuator with dynamic loads, DE196 48 545 A1 proposes the provision of three-dimensionally structured,electrically conductive electrodes between the metallization strips andthe outer contact which are connected to the metallization strips viapartial points of contact and are designed to be expandable between thepoints of contact.

On account of the increasingly confined space in the engine compartment,however, there is less and less room for the individual enginecomponents.

The object of the invention is therefore to improve a piezoelectricactuator according to the preamble of claim 1 in such a way that asecure electrical contact is ensured with the minimum amount of spacerequired, even with high dynamic loads.

The object is achieved by the features of the independent claim.Advantageous embodiments of the invention are characterized in thesubclaims.

The invention is characterized in that the outer electrodes comprise atleast one region which is embodied in such a way that it compensateslength variations of the piezoelectric actuator in the main oscillationdirection by means of elastic deformation as a result of its design andarrangement. The elastic deformation exclusively extends inside a planewhich is parallel to the main oscillation direction. Hereinafter thisplane is referred to as the oscillation plane.

The outer electrode, which is formed in such a way that it enables greatelastic deformation, ensures the reliable and durable contact of thepiezoelectric actuator. Because of the elastic deformation in each casein an area exclusively within a plane in each case which runs parallelto the main oscillation direction of the piezoelectric actuator, aminimal space requirement is achieved.

The areas of the outer electrode are here preferably arranged inparallel, with the minimum possible spacing, to the respective sidewalls of the piezoelectric actuator. This results in a particularlycompact type of construction. However, areas arranged differently inparallel to the main oscillation direction are also conceivable, forexample, in the case of semicircular outer electrodes encompassing thepiezoelectric actuator. The oscillation planes are produced by allplanes which touch the semicircle tangentially.

It is particularly advantageous if the outer electrode has a comb-shapedprofile with contact teeth. The contact teeth may be electroconductivelyconnected to the metallization strip in a simple manner, for example, bymeans of soldering, for contacting the inner electrodes. The comb-shapedprofile enables great elastic deformation between the individualsoldering points. Naturally, other profile shapes which enable a highlevel of elastic deformation in the main oscillation direction are alsoconceivable,. for example, a sinusoidal or triangular profile.

Exemplary embodiments of the invention are explained in more detailhereinafter with reference to the diagrams. The diagrams show:

FIG. 1 a piezoelectric actuator with elastically deformable outerelectrode,

FIG. 2 an outer electrode in a plane view,

FIG. 3 a curved outer electrode with adhesive layer on one side,

FIG. 4 a curved outer electrode with adhesive layer on both sides,

FIG. 5 a section through a piezoelectric actuator with an outerelectrode glued on.

Elements of the same construction and function have the same referencecharacters in all the figures.

FIG. 1 shows a piezoelectric actuator 1, comprising a stack of aplurality of individual piezoelectric actuator elements 2, 2′, 2″ whichare arranged between inner electrodes 3, 3′, 3″. The inner electrodes 3,3′, 3″are brought out of the stack in an alternating manner andelectrically connected in parallel. A first metallization strip 4 and asecond metallization strip 5 are attached to the outside of thepiezoelectric actuator 1 for this purpose. The inner electrodes 3, 3′,3″ are respectively connected in an alternating manner to the first orthe second metallization strips 4, 5. A first outer electrode 6 and asecond outer electrode 7 are fixed to the first or the secondmetallization strips 4, 5 respectively preferably by means of solderingin order to electrically contact the piezoelectric actuator. The outerelectrodes 6, 7 are connected with a first connection element 8 and asecond connection element 9 for externally contacting the piezoelectricactuator 1. If an electric voltage is applied to the outer electrodes 6,7, depending on the polarity of the voltage, the piezoelectric actuatorelements expand in the direction of the field or they contract. Withalternating polarity, the piezoelectric actuator thus executes afundamental oscillation in the direction of the field.

The outer electrodes 6, 7 are preferably designed as comb-shaped piecesand attached to the piezoelectric actuator 1 in such a way that theyexhibit their greatest elasticity in the main oscillation direction 10of the piezoelectric actuator 1. In the figure the shaped pieces 6, 7are curved for better mechanical location in such a way that they arefixed to two sides of the piezoelectric actuator 1 in each case.

FIG. 2 shows an outer electrode 6 in the shape of a comb in a planeview. The electrical contacting of the outer electrode 6 takes place viaa connection element 8. The connection element 8 is connected to awave-form region 16 of the outer electrode 6 for this purpose. Thewave-form region 16 is embodied as a conductor plate, from whichindividual contact teeth 11 running in parallel to one another leadaway. As a result of the wave-form embodiment of the conductor plate, ahigh degree of elasticity is achieved in this region. The contact teeth11 form a straight end 12. The contact teeth 11 are fixed to themetallization strip 4, preferably by means of soldering, in order toelectrically contact the inner electrodes 3, 3″, in the region of thestraight end 12.

The current is introduced via the connection element 8 into thewave-form part 16 of the outer electrode 6 and flows via the individualcontact teeth 11 and the metallization zone 4, 5 to the inner electrodes3, 3″. At the point of contact between the wave-form part 16, the outerelectrode 6 and the connection element 8 the entire current isintroduced into the outer electrode 6. For this reason, the wave-formpart 16 has a larger cross-section at this point. As the flow of currentin the wave-form part 16 of the outer electrode 6 constantly decreasesalong the principal axis 18 with increasing distance from the connectionelement 8, the cross-section of the wave-form part 16 preferably tapersalong its principal axis 18. The tapering form of the wave-form part 16of the outer electrode 6 further increases elasticity in this region.

The outer electrode 6 can preferably be manufactured from a bronze alloy(e.g. CuSn6) using etching technology.

FIG. 3 shows a section through the outer electrode 6. The outerelectrode 6 is fixed mechanically to the piezoelectric actuator 1 bymeans of adhesive 14. Preferably the outer electrode 6 is bent at anangle a for fixing to the piezoelectric actuator 1, parallel to thefirst, straight end 12 of the contact teeth 11. Preferably the angle αis <90°. As a result of this, when the outer electrode 6 is pressedagainst the right-angled piezoelectric actuator 1 an elastic forcearises which ensures that the contact teeth 11 of the outer electrode 6abut the piezoelectric actuator 1 during the gluing process. The contactteeth 11 are soldered on in a region around the straight end 12 to themetallization zone 4 for electrical contact. This region is left clearwhen the adhesive 14 is applied. In order to ensure flat abutment on themetallization strip 4 in the region of the straight end 12 of thecontact teeth 11, an additional offset 20 is provided which evens outthe adhesive layer 14 between the shaped piece 6 and the piezoelectricactuator 1.

The thickness of the layer of adhesive 14 is determined by the admixtureof particles of a preselected size. The adhesive 14 is composed in sucha manner that it ensures electric insulation between the outer electrode6 on the one hand and the piezoelectric actuator elements 2, 2′, 2″ andthe inner electrodes 3, 3′, 3″ on the other hand. Thus, no additionallayer of insulation is necessary, further reducing the requirement forconstruction space. The adhesive is permanently elastic in order to alsoensure secure fixing of the outer electrode 6 during operation of thepiezoelectric actuator 1.

As shown in FIG. 3, the adhesive 14 can be applied on one side on theinside of the shaped piece 6 or on both sides, as shown in FIG. 4. Theapplication of the adhesive 14 on both sides results in additionalinsulation of the outer electrode 6 from the external environment.

In FIGS. 2-4 only the structure of the first outer electrode 6 isdescribed in each case. The second outer electrode 7 is identical instructure, however, so that an additional description is unnecessary.

FIG. 5 shows a section through the piezoelectric actuator 1 with theglued-on outer electrodes 6, 7. The section is perpendicular to the mainoscillation axis 10 of the piezoelectric actuator 1. The respectivestraight sections B₁-B₈ of the first and second outer electrode 6, 7respectively represent a region of the outer electrodes which cancompensate length variations of the piezoelectric actuator 1 by means ofelastic deformation by virtue of their form and arrangement. The elasticdeformation of the respective regions B₁-B₈ runs respectively within aplane perpendicular to the plane of the image, characterized by therespective segmented line, and parallel to the main oscillationdirection 10.

After gluing on the outer electrodes 6, 7 and soldering in the region ofthe straight end 12, 12′ of contact teeth 11, 11′, the piezoelectricactuator 1 is covered in a continuous layer of adhesive 22. Thecontinuous layer of adhesive 22 can be applied, for example, by means ofimmersion. This gives the piezoelectric actuator 1 full electricprotection from the environment. A further advantage of this coatingwith adhesive is that when a fuel-resistant adhesive is used, the wholepiezoelectric actuator 1 can be made fuel-resistant. This isparticularly advantageous as the piezoceramic only has slight resistanceto moisture. As a result of slight resistance to moisture, moisture canpenetrate to the inner electrodes 3, 3′, 3″, which would result in adeterioration of the dielectric properties.

1.-12. (canceled) 1.-10. (canceled)
 13. A piezoelectric actuator,comprising: a stack of a plurality of individual piezoelectric actuatorelements disposed between inner electrodes and selectively contractingand expanding along a main oscillation direction in dependence on anapplied electric voltage; first and second metallization stripsalternatingly connected with said inner electrodes; first and secondouter electrodes respectively fixed to said first and secondmetallization strips for electrically contacting the piezoelectricactuator; said outer electrodes having at least one region configuredfor compensating for length variations of the piezoelectric actuator ina main oscillation direction by an elastic deformation thereofsubstantially exclusively in a plane parallel to the main oscillationdirection, said outer electrodes having a comb-shaped profile with ameander-form conductor plate and contact teeth for contacting saidmetallization strips projecting from said conductor plate; and first andsecond connection elements respectively connected to said first andsecond outer electrodes for externally contacting the piezoelectricactuator.
 14. The piezoelectric actuator according to claim 13, whereinsaid meander-form conductor plate is tapered along a principal axisthereof.
 15. The piezoelectric actuator according to claim 13, whereinsaid contact teeth extend parallel to one another and have the samelength at a first end, and said contact teeth at said first end aresoldered to said metallization strips for establishing an electricalcontact.
 16. The piezoelectric actuator according to claim 13, whereinsaid outer electrodes are bent by an angle α<90° parallel to a first,straight end region of said contact teeth, for fixing to thepiezoelectric actuator.
 17. The piezoelectric actuator according toclaim 13, wherein said outer electrodes are mechanically fixed to thepiezoelectric actuator by way of an adhesive and said contact teeth areleft free of the adhesive for soldering to said metallization strips.18. The piezoelectric actuator according to claim 17, wherein saidadhesive is configured and disposed to ensure electric insulationbetween said outer electrodes on one side and said piezoelectricactuator elements and said inner electrodes on the other side.
 19. Thepiezoelectric actuator according to claim 17, wherein a thickness of alayer of adhesive between said outer electrodes on one side and saidpiezoelectric actuator elements and said inner electrodeson the otherside is determined by an admixture of particles of a preset size. 20.The piezoelectric actuator according to claim 17, wherein said adhesiveis a fuel-resistant adhesive.
 21. The piezoelectric actuator accordingto claim 13, which comprises an adhesive completely covering thepiezoelectric actuator.
 22. The piezoelectric actuator according toclaim 13, wherein said outer electrodes are formed of an etched bronzealloy.